bug#70408: 30.0.50; Eglot and Project integration

[Dmitry Gutov <dmitry@gutov.dev>]

Hi Joao,

On 16/04/2024 15:33, João Távora wrote:
> On Tue, Apr 16, 2024 at 12:42 PM Eli Zaretskii<eliz@gnu.org>  wrote:
> 
>> I think this discussion should include João, so I added him.
> Alright.
> 
> More importantly, I think this discussion should include Dmitry, as this seems
> to me a project.el extension.
> 
> This discussion should be aware of these half-recent developments
> https://github.com/joaotavora/eglot/discussions/1337
> 
> In a few words, Eglot user's main gripe with project.el is project.el's
> inability to help the user define or designate subprojects within
> larger projects.

IIUC Ergus's request is primarily about a situation where an 
"out-of-tree" build is used. Meaning, the directory for build artefacts 
is not a subdirectory of the project root, but -- apparently -- some 
sibling directory of it (e.g. "../build"). So it's somewhat atypical, 
although I suppose the solution from the link above might work with it too.

This bug is split off from an emacs-devel discussion, where I posted a 
draft solution of mine: 
https://lists.gnu.org/archive/html/emacs-devel/2024-04/msg00279.html

I'm curious for any feedback - like would that be good enough for this 
and related cases, or maybe if someone has an even simpler approach in mind.